The present invention relates to a picture data recording and/or reproducing apparatus for recording and/or reproducing data of a bitstream compressed in accordance with a MPEG standard and, more particularly, to such apparatus operable for providing slow speed reproduction of the recorded bitstream data from a tape recording medium.
In a digital video tape recorder (hereinafter, a digital VTR), high definition picture data, which is compressed or encoded in accordance with a predictive coding technique, may be recorded/reproduced on or from a magnetic tape by utilizing a rotary head. Such predictive encoding technique may comply with a so-called MPEG (Motion Picture Experts Group) standard.
In a MPEG standard or system, data or information is encoded in encoding units or so-called macro-blocks. For each such macro-block, the information may be compressed by temporal compression in accordance with temporal inter-picture correlation and spatial compression utilizing a discrete cosine transform (DCT) and subsequently encoded by variable length encoding or Huffman code.
Further, in the MPEG system, the information or data may be arranged in pictures which correspond to video frames. As hereinafter more fully described, each of these pictures may be either an intra-coded (I) picture, an inter-frame predictively coded (P) picture or a bi-directionally predictive coded (B) picture and such pictures may be arranged in a predetermined sequence. A plurality of such pictures may be further arranged into a so-called group-of-pictures (GOP). Each such GOP includes at least one intra-frame coded (I) picture. Such GOP arrangement or structure may be utilized to enable or facilitate random accessing of a desired picture within a respective GOP.
Furthermore, during encoding in the MPEG system, a memory, which may be a random-access memory (RAM) or a read-only memory (ROM), may be utilized. Such use of the memory may facilitate in the forming of a picture, such as a predicted picture. Such predicted or "future" picture (this picture may lie ahead of a current picture by one or more pictures) is formed based upon an intra-frame coded picture, and the space in-between is filled with bi-directionally coded pictures. Such bi-directional prediction enables a relatively high compression ratio to be achieved, although additional encoding time may be required. On the other hand, during decoding, real-time processing is utilized.
The MPEG encoding system is not limited to only a predetermined picture size or resolution, but may be applied to a number of systems. For example, the MPEG encoding system may be applied to the NTSC and PAL systems.
Picture data, which is encoded in accordance with the MPEG system, may be recorded onto tracks of the recording medium by utilizing a so-called inclined azimuth recording system. In such recording system, data may be recorded/reproduced into/from two neighboring tracks by two heads having gap directions which are inclined relative to each other. Encoded picture data corresponding to one frame may be recorded across a plurality of tracks on the recording medium or magnetic tape.
In reproducing data from a track with the above digital VTR, an output or reproduced data is obtained only from the head having an inclination angle of the gap direction (or azimuth) which is coincident with that utilized in recording the data. In other words, data is not reproduced from the head having the non-coincident azimuth angle. As such, the tape feed stop position is controlled so as to cause the two heads to respectively trace the tracks having the coincident azimuth angle such that the two heads alternatively reproduce playback signals therefrom.
In the above digital VTR, the heads sequentially scan the track patterns during a normal-speed reproduction mode. However, during a varying-speed reproduction mode, the heads obliquely scan the track patterns on the magnetic tape. The varying-speed reproduction may be obtained by changing the running speed of the magnetic tape. For example, a time to tape displacement ratio may be 1:3 and 3:1 for triple speed reproduction and 1/3 slow speed reproduction, respectively, as compared to 1:1 for normal speed reproduction.
If a slow speed reproduction is performed with the above digital VTR, data may be read out discontinuously. For example, consider the situation in which picture data which is compression encoded in accordance with the above-described MPEG system is to be reproduced with a slow speed. In such situation, since the amount of data available to be read out per unit time may be decreased, it may be necessary to increase the amount of read or sent-out data so as to match the bit rate or period to the original bit rate or period. To increase the amount of such data, "0"s may be simply inserted or stuffed therein. As a result, "0"s may be added between portions of data, thereby producing a discontinuous or interrupted boundary or boundaries of readout data as, for example, illustrated in FIG. 15. As shown therein, "0"s are inserted or stuffed between portions of I and P picture data which are readout from the tape in each scan or trace of the heads so as to effectively match the bit rate to the original bit rate. (As hereinafter more fully described, a picture header (PH) precedes the data of each picture.) During the decoding of such data by a MPEG decoder, the stuffed "0"s may be mistakenly processed as "normal" image data. Accordingly, the MPEG decoder may incorrectly decode the original data such that the picture is disrupted during slow-speed reproduction.
As is to be appreciated, it would be desirable to correctly decode picture data which is encoded by a predictive coding technique, such as that which complies with a MPEG standard, during slow speed reproduction.